Senator Jeff Merkley of Oregon has been getting a question from constituents: does he support holding a vote on the bill to impose new sanctions on Iran that’s being championed by some Senate Dems and Republicans? The question in part is fueled by lobbying from outside groups on both sides of the issue.

Merkley has now responded with a letter to constituents in which he comes out against holding a vote on the sanctions bill right now. Here’s the key quote from the letter, which was passed along by a source:

At this time I do not support additional sanctions legislation because I share the views of many foreign policy experts that it could undermine the ongoing negotiations and weaken our multinational coalition, ultimately making less likely our goal of preventing Iran from acquiring nuclear weapons.

Merkley’s caution — and the silence among Dems in general — underscores how carefully Dems are approaching the domestic politics of engagement with Iran right now. This, even though majorities support the nuclear deal with Iran and even though Obama has asked Democrats for the room to make negotiations work. As Ron Fournier put it: “you’d expect fellow Democrats to give him the benefit of the doubt.”

The current count right now is that 58 or 59 Senators — including 16 Dems — support the bill, putting it within range of passing and even overriding a presidential veto. On the other side, 10 Dem committee chairs have come out against it, and Harry Reid is said to be holding off a vote on it.

One question right now is what to make of the silence among Democrats who have not declared a position. Some, such as Peter Beinart, argue that the lack of pressure on Senators to take a public position might end up making it more likely that such a bill passes.

“How can so many prominent Democratic senators support a bill so widely scorned by Democratic foreign policy experts?” Beinart asked the other day. “The more these senators are forced to publicly defend their positions on Iran, the more politically costly those positions will become. What we don’t know is whether the left can generate a movement strong enough to force that public debate.”

It’s also possible, however, that the silence among Dems bodes well for the White House: it could mean they’re holding off from supporting the sanctions bill, and laying low while doing so to avoid political heat from the bill’s proponents.

On another front, the Wall Street Journal reports that House GOP leaders may bring the Menendez bill to the floor. moving it forward in another chamber. The Huffington Post makes a good point about this, noting that the increasing involvement of House Republicans could make it more likely that Senate Dems come out against it, because it could inject a heavy dose of partisanship into what had been a bipartisan affair.”

Indeed, a plausible case can be made that Republicans want to see the sanctions bill move forward in part because it will exacerbate the already-deepening rift among Democrats over Iran and make it more likely that Obama will have to veto something that passed both houses with large bipartisan majorities. Whatever the motive of Senate Dems who are supporting this bill, they are helping Republicans drive this wedge.

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UPDATE: I just got off the phone with Senator Chris Murphy of Connecticut, who also tells me he does not support moving forward with the sanctions bill at this time.

“I won’t be signing on to it,” Murphy told me. “I think it risks blowing up the negotiations or at the very least allows for countries like Russia and China to start weakening existing sanctions.”

Murphy suggested he didn’t see why Congress could not pass a sanctions bill in six months, if the negotiations break down, rather than passing one right now that is set to take hold in six months — a step the White House fears will derail negotiations.

“I think there is no doubt Congress could rally within days of any breakdown in negotiations to pass a sanctions bill,” he said. “I stand ready to impose new sanctions the minute these negotiations fall apart. I just don’t see the need to pass a bill today that would be in violation of the temporary agreement that the P5+1 signed with Iran.”

“I have great respect for my colleagues that believe that this could be a helpful Sword of Damocles over these negotiations,” he added. “I just trust the administration in their belief that this could have a really deleterious effect.”

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.

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